A crystal oscillator is known which includes a sub oscillator having sub electrodes provided separately from main electrodes on a crystal substrate.
[Patent Document 1] Japanese Laid-open Patent Publication No. 2011-135342
[Patent Document 2] Japanese Laid-open Patent Publication No. 2013-106052
However, according to the conventional technique as described above, it is difficult to detect a state of the crystal oscillator before a transition to an output stop state (for example, clock stop) based on a signal appearing in the sub electrodes. Such an output stop may occur suddenly due to abnormality of the crystal oscillator or the like.